Iontophoresis has in the past been defined as "the introduction, by means of electric current, of ions of soluble salts into the tissues of the body for therapeutic purposes." Iontophoretic devices for delivering ionized drugs through the skin have been known since the early 1900's. Deutsch UK Patent No. 410,009 (1934) describes an iontophoretic device which overcame one of the disadvantages of such early devices, namely that the patient needed to be immobilized near the source of electric current. The Deutsch device was powered by a galvanic cell formed from the electrodes and the material containing the drug to be delivered transdermally. The galvanic cell produced the current necessary for iontophoretically delivering the drug. This device thus allowed the patient to move around during iontophoretic drug delivery and thus imposed substantially less interference with the patient's daily activities.
Today, iontophoresis is not limited solely to the delivery of ions (e.g., drug ions) into the body by means of electric current. For example, it is now recognized that iontophoretic delivery devices can be used to deliver an uncharged drug or agent into the body. This is accomplished by a process called electroosmosis. Electroosmosis is the transdermal flux of a liquid solvent (e.g., the liquid solvent containing the uncharged drug or agent) which is induced by the presence of an electric field imposed across the skin by the donor electrode. As used herein, the terms "iontophoresis" and "iontophoretic" refer to (1) the delivery of charged drugs or agents by electromigration, (2) the delivery of uncharged drugs or agents by the process of electroosmosis, (3) the delivery of charged drugs or agents by the combined processes of electromigration and electroosmosis, and/or (4) the delivery of a mixture of charged and uncharged drugs or agents by the combined processes of electromigration and electroosmosis.
Recently, a number of United States patents have issued in the iontophoresis field, indicating a renewed interest in this mode of drug delivery. For example, Vernon et al U.S. Pat. No. 3,991,755; Jacobsen et al U.S. Pat. No. 4,141,359; Wilson U.S. Pat. No. 4,398,545; and Jacobsen U.S. Pat. No. 4,250,878 disclose examples of iontophoretic devices and some applications thereof. The iontophoresis process has been found to be useful in the transdermal administration of medicaments or drugs including lidocaine hydrochloride, hydrocortisone, fluoride, penicillin, dexamethasone sodium phosphate, insulin and many other drugs. Perhaps the most common use of iontophoresis is in diagnosing cystic fibrosis by delivering pilocarpine salts iontophoretically. The pilocarpine stimulates sweat production; the sweat is collected and analyzed for its chloride content to detect the presence of the disease.
In presently known iontophoretic devices, at least two electrodes are used. Both of these electrodes are disposed so as to be in intimate electrical contact with some portion of the skin or other membrane surface of the body. One electrode, called the active or donor electrode, is the electrode from which the ionic substance, medicament, drug precursor or drug is delivered into the body by iontophoresis. The other electrode, called the counter or return electrode, serves to close the electrical circuit through the body. In conjunction with the patient's skin contacted by the electrodes, the circuit is completed by connection of the electrodes to a source of electrical energy, e.g., a battery. For example, if the ionic substance to be delivered into the body is positively charged (i.e., a cation), then the anode will be the active electrode and the cathode will serve to complete the circuit. If the ionic substance to be delivered is negatively charged (i.e., an anion), then the cathode will be the active electrode and the anode will be the counter electrode.
Alternatively, both the anode and cathode may be used to deliver drugs of opposite charge into the body. In such a case, both electrodes are considered to be active or donor electrodes. For example, the anode can deliver a positively charged ionic substance into the body while the cathode can deliver a negatively charged ionic substance into the body.
Iontophoretic delivery devices generally require a reservoir or source of the beneficial agent to be iontophoretically delivered or introduced into the body. Examples of such reservoirs or sources of agents include a pouch or cavity as described in the previously mentioned Jacobsen U.S. Pat. No. 4,250,878, a porous sponge or pad as disclosed in Jacobsen et al U.S. Pat. No. 4,141,359, or a pre-formed gel body as described in Webster U.S. Pat. No. 4,383,529 and Ariura et al U.S. Pat. No. 4,474,570. Such drug reservoirs are electrically connected to the anode or the cathode of an iontophoresis device to provide a fixed or renewable source of one or more desired agents.
Iontophoretic delivery devices which are attachable at a skin surface and rely on electrolyte fluids to establish electrical contact with such skin surfaces can be divided into at least two categories. The first category includes those devices which are prepackaged with the liquid electrolyte contained in the electrode receptacle. The second type of device uses dry-state electrodes whose receptacles are customarily filled with a drug/electrolyte solution immediately prior to application to the body. With both types of devices, the user currently experiences numerous problems which make their use both inconvenient and problematic.
With respect to the prefilled device, storage is a major concern. Many drugs have poor stability when in solution. Accordingly, the shelf life of prefilled iontophoretic drug delivery devices is unacceptably short. Corrosion of the electrodes and other electrical components is also a potential problem with prefilled devices. For example, the return electrode assembly usually contains an electrolyte salt such as sodium chloride which over time can cause corrosion of metallic and other electrically conductive materials. Another problem with prefilled electrodes concerns maintaining the sterility of the electrodes and preventing microbial growth therein. This is a particular problem when the liquid used to conduct iontophoresis is water. Although antimicrobial agents can be added to the drug and/or electrolyte reservoirs of an iontophoretic delivery device, the addition of such agents tends to compromise the efficiency of agent delivery. Leakage is another serious problem with prefilled iontophoretic drug delivery devices. Leakage of drug or electrolyte from the electrode receptacle can result in an inoperative or defective state. Furthermore, such prefilled devices are difficult to apply because the protective seal which covers the electrode opening and retains the fluid within the receptacle cavity is must be removed prior to application on the skin. After removal of this protective seal, spillage often occurs in attempting to place the electrode on the skin. Such spillage impairs the desired adhesive contact of the electrode to the skin and also voids a portion of the receptacle cavity. The consequent loss of drug or electrolyte solution can disrupt electrical contact with the electrode and otherwise disrupts the desired uniform potential gradient applied by these devices.
Although dry-state electrodes have numerous advantages in ease of storage, several problems remain. For example, the drug and electrolyte receptacles of such a device are conventionally filled through an opening prior to application of the device to the patient's skin. Therefore, the same problem of spillage and loss of drug or electrolyte upon application occurs as with the prefilled electrode.
Frequently, such electrodes are not well structured to develop the proper uniform current flow required in iontophoresis applications. Such nonuniform current flow may result from the occurrence of air pockets within the receptacle cavity at the skin surface. Such effects are particularly troublesome in iontophoresis applications, where a nonuniform current distribution may result in excessive skin irritation or "burning".
More recently, iontophoretic delivery devices have been developed in which the donor and counter electrode assemblies have a "multilaminate" construction. In these devices, the donor and counter electrode assemblies are each formed of multiple layers of (usually) polymeric matrices. For example, Parsi U.S. Pat. No. 4,731,049 discloses a donor electrode assembly having hydrophilic polymer based electrolyte reservoir and drug reservoir layers, a skin-contacting hydrogel layer, and optionally one or more semipermeable membrane layers. In addition, Ariura et al U.S. Pat. No. 4,474,570 discloses a device wherein the electrode assemblies include a conductive resin film electrode layer, a hydrophilic gel reservoir layer, an aluminum foil conductor layer and an insulating backing layer.
The drug and electrolyte reservoir layers of iontophoretic delivery devices have typically been formed of hydrophilic polymers. See for example, Ariura et al, U.S. Pat. No. 4,474,570; Webster U.S. Pat. No. 4,383,529 and Sasaki U.S. Pat. No. 4,764,164. There are several reasons for using hydrophilic polymers. First, water is biocompatible, highly polar and therefore a preferred solvent for many drugs. Secondly, hydrophilic polymer components (i.e., the drug reservoir in the donor electrode and the electrolyte reservoir in the counter electrode) can be hydrated while attached to the body by absorbing water from the skin or from a mucosal membrane. For example, skin contacting electrodes can be hydrated by absorbing sweat or water from transepidermal water loss. Similarly, electrodes attached to an oral mucosal membrane can be hydrated by absorbing saliva. Once a sufficient quantity of water is sorbed into the drug and electrolyte reservoirs, ions are able to move through the reservoirs and across the tissue, enabling the device to deliver agent to the body. Hydrogels have been particularly favored for use as the drug reservoir matrix and electrolyte reservoir matrix in iontophoretic delivery devices, in part due to their high equilibrium water content and their ability to absorb water from the body. In addition, hydrogels tend to have good biocompatibility with the skin and with mucosal membranes. However, since many drugs and certain electrode components are unstable in the presence of water, iontophoretic drug delivery devices having a drug reservoir formed of a prehydrated hydrogel may also have an unacceptably short shelf life. One solution to the drug stability problem is to use hydrophilic polymer drug and electrolyte reservoirs which are in a substantially dry state, i.e., in a non-hydrated condition. The drug and/or electrolyte can for example be dry blended with the hydrophilic polymer and then cast or extruded to form a non-hydrated, though hydratable, drug or electrolyte containing reservoir. Unfortunately, the non-hydrated hydrophilic polymer components must first absorb sufficient quantities of water from the body before the device can operate to deliver drug. This delivery start-up period can take in excess of several hours. This delay makes many devices unsuited for their intended purpose. For example, when using an iontophoretic delivery device to apply a local anesthetic in preparation for a minor surgery (e.g., surgical removal of a mole), the surgeon and the patient must wait until the drug and electrolyte reservoirs of the delivery device become sufficiently hydrated before the anesthetic is delivered in sufficient quantities to induce anesthesia. Similar delays are encountered with other drugs.
In response to the difficulty of iontophoretic delivery of a drug which is unstable in water, Konno et al in U.S. Pat. No. 4,842,577 disclose in FIG. 4 an iontophoretic electrode assembly having a substantially non-hydrated drug containing matrix and a separate water reservoir which is initially sealed, using a foil sheet, from the drug containing portions of the electrode. In order to activate the Konno et al electrode assembly, the top of the water reservoir container is depressed, causing the foil sheet to break and thereby release the water into the non-hydrated drug-containing matrix. Unfortunately, this electrode design is not only difficult to manufacture but also is subject to severe handling restrictions. In particular, there is a tendency for the foil seal to be inadvertently broken during manufacture, packaging and handling of the electrode. This can have particularly drastic consequences especially when the seal is broken during manufacture or shipping of the device. Once the seal is broken, water is wicked into the drug-containing reservoir which can cause degradation of the drug and/or other components before the device is ever used.
Another disadvantage of using non-hydrated hydrophilic polymer components is that they have a tendency to delaminate from other parts of the electrode assembly during hydration. For example, when utilizing a drug reservoir matrix or an electrolyte reservoir matrix composed of a hydrophilic polymer, the matrix begins to swell as it absorbs water from the skin. In the case of hydrogels, the swelling is quite pronounced. Typically, the drug or electrolyte reservoir is in either direct contact, or contact through a thin layer of an ionically conductive adhesive, with an electrode. Typically, the electrode is composed of metal (e.g., a metal foil or a thin layer of metal deposited on a backing layer) or a hydrophobic polymer containing a conductive filler (e.g., a hydrophobic polymer loaded with carbon fibers and/or metal particles). Unlike the hydrophilic drug and electrolyte reservoirs, the electrodes do not absorb water and do not swell. The different swelling properties of the hydrophilic reservoir and the electrode, or the ionically conductive adhesive, results in shearing along their contact surfaces. In severe cases, the shearing can result in the complete loss of electrical contact between the electrode and the drug/electrolyte reservoir resulting in an inoperable device.